The present invention relates to a method and apparatus for generating power utilizing pressure-retarded osmosis. More particularly, the present invention is directed to variations and improvements in the method and apparatus disclosed in our prior U.S. Pat. No. 3,906,250.
The above patent discloses a method and apparatus for producing energy from the free energy of mixing by utilizing pressure-retarded osmosis. More particularly, the method and apparatus are characterized by the steps of, and means for, introducing at a high hydraulic pressure a first liquid, constituting a concentrated solution, of a high osmotic pressure into a first pathway which is at least partially defined by one face of a semi-permeable membrane; introducing at a lower hydraulic pressure a second liquid, constituting a dilute solution, of a lower osmotic pressure into a second pathway which is at least partially defined by the opposite face of the semi-permeable membrane; maintaining the hydraulic pressures of the liquids on the opposite faces of the membrane at a pressure difference which is less than the osmotic pressure difference between the liquids to affect, by Pressure-Retarded-Osmosis (hereinafter sometimes referred to as "PRO"), the passage of at least a part of the dilute solution through the semi-permeable membrane, forming a pressurizing mixed solution of greater volume than the concentrated solution introduced into the first pathway; and converting the potential energy stored in the pressurized mixed solution to useful energy.
In the preferred embodiments of the invention described in the above patent, the potential energy stored in the mixed solution is converted by depressurizing the mixed solution, the concentrated solution having been pressurized before being introduced into the first pathway through the semi-permeable membrane. As an example, the converting means could be a hydroturbine generating electrical power. Thus, the net output of energy from the system described would be equal to the difference between the output from the hydroturbine and the input to the pressurizing pump for pressurizing the concentrated solution.
If pressurizing pumps and hydroturbines were to have 100% efficiency, this pressurization-depressurization sequence of the concentrated solution would have no effect on energy production. However, in practice, all pumps and hydroturbines have less than 100% efficiency, and therefore considerable energy is wasted in pressurizing the concentrated solution and depressurizing the mixed solution when only a small volume of the mixed solution (that permeated through the semi-permeable membrane) contributes to the net output generated. This drawback is made even more serious to lowcost power generation by the use of the PRO technique since, on the one hand, the mechanical efficiency decreases with increasing pressure of the concentrated solution, and on the other hand, energy costs decrease with increasing pressure.